The company is only just beginning to roll out this feature after testing the horn inside the car itself; engineers didn't want the vehicle to blast away at other people unnecessarily or by mistake. So when the car accidentally honked at someone during testing, only the Googlers on the inside could hear it and correct for it by teaching the computer that that wasn't an appropriate time to honk.
Although the system will likely require a lot more fine-tuning, it's easy to imagine someday that Google's honking will be even more reliable and understandable than human honking. Think about your last car ride; did someone use their horn on you? Did it happen because there was truly an immediate and urgent danger, or was it just because the person behind the wheel was having a bad day? Sometimes, it's hard to tell.
But Google appears to want its driverless car to be more judicious with its use of the horn.
"If another vehicle is slowly reversing towards us, we might sound two short, quieter pips as a friendly heads up to let the driver know we’re behind," the company wrote in its latest monthly driverless car report. "However, if there’s a situation that requires more urgency, we’ll use one loud sustained honk."